How to stop blender from generating duplicate vertices on export?
It is incorrect to assume the number of glTF verts will equal the number of Blender verts. Each glTF vert has a unique normal for example, while each Blender vert can have a different normal for each poly it is in. Your model uses flat shading and you can see each vertex has multiple normals.

To handle this there are &quot;duplicate&quot; glTF verts (that have the same position, but different normals). The same thing happens for all other &quot;vertex per poly&quot; data, like UVs.
&quot;Duplicate&quot; verts are inserted only when necessary, so turn off export of anything you don't need—normals, tangents, UVs, vertex colors, materials—to get a smaller glTF vert count.